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Wed, 28 Oct 2026 04:33:20 +0000 X-Mizu-Trace-ID: bee4a2cb7972e2c2 X-Migadu-Flow: FLOW_OUT Message-ID: Date: Wed, 28 Oct 2026 12:32:54 +0800 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 32/46] mm/memory_hotplug: avoid races in pfn_to_online_page() with dying pgmap To: "David Hildenbrand (Arm)" Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, qi.zheng@linux.dev, shakeel.butt@linux.dev, ljs@kernel.org, kasong@tencent.com, baohua@kernel.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, baoquan.he@linux.dev, baolin.wang@linux.alibaba.com, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kees@kernel.org, peterz@infradead.org, jannh@google.com, pfalcato@suse.de, hughd@google.com, pasha.tatashin@soleen.com, pratyush@kernel.org, willy@infradead.org, jack@suse.cz, ziy@nvidia.com, riel@surriel.com, harry@kernel.org, apopple@nvidia.com, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, usama.arif@linux.dev, kas@kernel.org, matthew.brost@intel.com, jhubbard@nvidia.com, peterx@redhat.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, osalvador@suse.de, minchan@kernel.org, muchun.song@linux.dev, xu.xin@linux.dev, chengming.zhou@linux.dev, chrisl@kernel.org, shikemeng@huaweicloud.com, nphamcs@gmail.com, youngjun.park@lge.com, gourry@gourry.net, joshua.hahnjy@gmail.com, rakie.kim@sk.com, ying.huang@linux.alibaba.com, jgg@ziepe.ca, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, josef@toxicpanda.com References: <20261027-fixes_core-v1-32-6f74bec9a5bb@kernel.org> <20261027121307.40431-1-lance.yang@linux.dev> <87e3d64e-8b25-4d51-a984-7ab8a58f785d@kernel.org> Content-Language: en-US From: Lance Yang In-Reply-To: <87e3d64e-8b25-4d51-a984-7ab8a58f785d@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: B4703C0002 X-Rspam-User: X-Stat-Signature: gxfc7itzcpg73y3bmhkbyajafamxkjhd X-Spam-Status: No, score=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1793162002-461208 X-HE-Meta: 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 RaixzGF3 3dF2FVrAc3o2zJCu47uIv/ddbV+fQUJVMuq+t+vpTRSG4fIzmQqlwr6vk31Vc6UuXfBHhC8YiD9foHx5rWL+MFUg8CqcGOgUn2hsOL/7+6QlY3yHLaU/wGnTLhYvD8EEhPFiCwMRsBH5sOTnpMyf+QzQeGzji2NqufpOE7PoUwXLm8MExF17R7+J486ONp2Kr/vV1pz3w3ycpigwoNVX898OFbicrJuovSN3RyMPaxaw7jYhoLU0kewKajBw0dEvWGUyjSpLk+6S0Pwt8gXwSRBSBTV3zNqbbpH+XDALAP6Pu1QQO0A1wQM6ZHhoHaflQN0MVF9nu0QbvaqtfdAYrxP1wEtza0QMFFoQSkAsM6lUNW1+dPvvoMY9wiTpSl6D/clIHHu3Bjx2+JY4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 2026/10/28 03:41, David Hildenbrand (Arm) wrote: >> LLM stumbled upon a possible race in kpage_read() ... >> >> Say the reader passes pfn_section_valid() for a device PFN in a section >> shared with online RAM, gets preempted, and resumes after >> pgmap_array_delete(). We'd then find no pgmap and return pfn_to_page(pfn), >> even though the subsection is no longer valid ... > > I think that's similar to our pfn_to_online_page() vs. memory offlining races, > just slightly different. Getting these races down to 0 is pretty hard. > > This patch here tries to do better and arguably almost looks like a cleanup. > > I wouldn't worry about that one for now; it's extremely unlikely to hit. And it' Ack. >> >> Could we take the memory hotplug read lock in kpage_read(), covering both >> the lookup and the metadata reads? We can drop it before put_user(), once >> we're done with the page. >> >> Maybe along these lines? >> >> ---8<--- >> diff --git a/fs/proc/page.c b/fs/proc/page.c >> index 6aa99e42ec22..12bed36d76dc 100644 >> --- a/fs/proc/page.c >> +++ b/fs/proc/page.c >> @@ -12,6 +12,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> #include >> @@ -77,6 +78,8 @@ static ssize_t kpage_read(struct file *file, char __user *buf, >> count = min_t(unsigned long, count, (max_dump_pfn * KPMSIZE) - src); >> >> while (count > 0) { >> + /* Prevent memory removal while inspecting the memmap. */ >> + get_online_mems(); > get_online_mems() is the big hammer we try to avoid (or ideally remove at some > point). And it's most pfn_to_online_page() users that are affected. > > A seqcount in pfn_to_online_page() could detect concurrent pgmap removal and > retry from the start. But I'd rather delay something like that. Fair enough, let's leave it alone for now given how unlikely it is to hit :) > Thanks for taking a look! (Can I convince you to review some of the THP fixes? > pleeeeaaasseee :) ) Of coursssssssse! I'm already looking through the THP fixes :D Cheers, Lance